Overflowing toilet

I had the overflow running into the toilet from a Geberit concealed built in cistern so i called back the guy who fitted it and he said he would put in a new flush unit which should solve the problem, he put in a new flush unit and it did not solve the problem, he checked the incoming water pressure and it was 5.5 bar which he said was the problem, called in seven trent they said as far as they were concerned we needed to install a pressure reducing valve at the point of entry to the property, we said we had been advised that the pressure for this cistern should only be 2.5 bar, but reading comments on geberit site I only need to replace a washer under the flush cistern and this would solve the problem, and this would be alot cheaper as we would'nt need to take out all the built in units, which way do I go? Des